WO2000014820A1

WO2000014820A1 - A METHOD OF FORMING CoOOH and NiOOH IN A NiMH ELECTROCHEMICAL CELL AND AN ELECTROCHEMICAL CELL FORMED THEREBY

Info

Publication number: WO2000014820A1
Application number: PCT/US1999/020173
Authority: WO
Inventors: Deepika B. Singh
Original assignee: Moltech Power Systems, Inc.
Priority date: 1998-09-04
Filing date: 1999-09-02
Publication date: 2000-03-16
Also published as: US6270535B1; AU6242799A

Abstract

A formation procedure for a NiMH electrochemical cell is disclosed that significantly shortens the time required to fully form such a cell. The formation procedure includes a first step during which the cell is charged at a constant voltage of preferably 1.0 volt for approximately three hours. A second charging step is performed by applying either a constant charge current at a predetermined rate of C/3 for five hours or applying a constant voltage of 1.45 to 1.5 volts for five to nine hours. A third step may optionally be used whereby the cell is charged at a constant current of C/10 for about two hours. NiMH cells subjected to this formation procedure have a much greater percentage of the starting cobalt material in the positive electrode converted to CoOOH thereby improving the conductive matrix formed about the Ni(OH)2/NiOOH particles, which constitute the active material of the positive electrode. The disclosed inventive formation procedures result in higher utilization, higher cell capacities following long-term storage, or storage at high temperatures as well as improved capacity recovery following deep discharge.

Description

WO 00/14820 PCTtUS99t20l73

A METHOD OF FORMING CoOOH AND NiOOH IN

A NiMH ELECTROCHEMICAL CELL AND AN ELECTROCHEMICAL CELL FORMED THEREBY

BACKGROUND OF THE INVENTION The present invention generally relates to the formation of a nickel metal hydride (NiMH) electrochemical cell. More particularly, the present invention relates to the formation of a cobalt

(El) conductive matrix on and around particles of electxochemically active nickel compound in a positive electrode of a NiMH electrochemical cell.

NiMH cells, particularly those NiMH cells having foam positive electrodes, are well known for their high capacities and energy densities. In typical NiMH cells, the negative electrode is formed of a metal hydride while the positive electrode is formed of nickel hydroxide Ni(OH)2. The

NiMH cells are assembled in a discharged state. During charging, the Ni(OH)2 converts to nickel oxyhy dioxide (NiOOH), which converts back to Ni(OH upon discharge. Because Ni(OH)2 is not electrically conductive, an electrically conductive matrix must be formed around the Ni(OH)2 particles. A common conductive matrix used in such positive electrodes is made of cobalt oxyhy droxide (CoOOH). To form a conductive matrix of CoOOH around Ni(OH)₂ particles, spherical Ni(OH)2 particles and oxides of cobalt are physically mixed to form a paste. One cobalt oxide compound that may be included in the paste is CoO. The cobalt oxide compound dissolves in an alkaline electrolyte to form HC0O2^" ions, which surround the spherical Ni(OH)2 particles and re-precipitate as cobalt hydroxide (Co(OH». The conductive network of CoOOH is formed when the electrode is charged in accordance with a formation procedure. Relatively low charging rates are used for long periods of time to form the conductive network and to activate the Ni(OH to NiOOH. This can be time consuming and labor intensive since this formation procedure involves charging an assembled cell with a constant current of C/20 for thirty-two hours.

One function of the formation procedure is to convert all the Co(OH)2, which includes cobalt in its +2 valance state (i.e. , Co(Η)), to CoOOH, which includes cobalt in its +3 valance state (i.e. , Co(lII)). If the cobalt is not converted completely to CoOOH, other species of cobalt are formed such as C03O4, which is electrochemically inactive.

Also, the cobalt that is left behind in the +2 valance state dissolves in the alkaline electrolyte as HCoC ^" ions. The HC0O2^' ions are very mobile, and move to and through the separator and plate the separator or the metal hydride negative electrode. The unconverted HC0O2^' ions readily get converted to C03O4 in the presence of oxygen, which is obtained when the positive electrode self-discharges to form Ni(OH)2. The migration and conversion of the HC0O2^" ions cause soft shorts and a redistribution of cobalt in the negative electrode. Because C03O is electrochemically inert and acts as an insulator within the electrode, the formation of C03O reduces the charge and discharge efficiency of the positive electrode. Longer soak times (i.e. , the time during which the electrode is soaked in electrolyte so as to dissolve the CoO in high temperatures) accelerate this process.

Since part of the Co(II) is converted to C03O4, less Co(II) is available to form the Co(ILI) conductive matrix during the formation procedure. Due to the detrimental effects of having C03O4 form in the positive electrode, a method is needed for foπ ing a NiMH cell that maximizes the percentage of Co(II) converted and maintained as Co(LlI).

Another function of the formation procedure is to activate as much of the Ni(OH)2 as possible in the positive electrode. In many cases the Ni(OH)2 at the outer surface of the electrode is charged to form NiOOH. This is followed by oxygen evolution. Once the oxygen evolution begins, the elec_trode no longer accepts charge to form the remaining Ni(OH)^j, wliich may be buried deep in the electrode. Activating as much of the Ni(OH>2 as possible not only influences the first cycle capacity, but also affects the performance of the cells during consecutive cycles. Therefore, there further exists the need for a formation procedure that maximizes the amount of Ni(OH)₂ that is

activated.

Another practical problem relating to the manufacture of NiMH cells results from storage of such cells for long periods of time after formation. When NiMH cells are stored for periods of up to nine months, they exhibit significant losses in capacity on the order of 10.5 percent of their initial capacity following formation. This loss in cell capacity is irreversible. Also, if cells are stored even for relatively short periods of time at relatively high temperatures, the cells similarly exhibit irreversible losses in capacity. Because long-term storage and storage at high temperatures following formation are difficult to avoid due to handling by third-party retail sales outlets and consumers, there exists a need for improved NiMH cells that do not exhibit such significant irreversible capacity loss under these somewhat common conditions. γ_et ano her problem occurs when NiMH cells are deeply discharged below recommended levels, as is the case when a consumer fails to turn off a device powered by a battery pack containing the NiMH cells. Following such deep discharges, the cells experience a significant and irreversible loss in capacity. Such losses may be approximately 10 percent of the cell capacity.

SUMMARY OF THE INVENTION Accordingly, it is an aspect of the present invention to provide a formation procedure for a

NiMH cell that converts a higher percentage of the Cofll) to Co(IH) to thereby minimize the detrimental effects of the formation of C03O . An additional aspect of the present invention is to provide a foπnation procedure that also enables a high percentage of the Ni(OH to be activated to form NiOOH. Yet another aspect of the present invention is to provide a formation procedure that not only converts a higher percentage of the Co(II) to Co(III), but also is performed in a fraction of the time previously required for the formation of a NiMH cell.

To achieve these and other aspects and advantages, the method of forming a NiMH electrochemical cell according to the present invention comprises the steps of. (a) providing an assembled electrochemical cell having a positive electrode including a mix of a cobalt(TI) compound and Ni(OH)2, and.(b) charging the cell at a constant voltage.

These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

Fig. 1 is a bar graph showing the percent of Co(IT) converted to Co(III) during formation using two conventional formation procedures and the formation procedure according to the present invention;

Fig. 2 is a line graph illustrating the percent of Co(H) converted to Co(III) as a function of charging current;

Fig. 3 is a bar graph comparing the percent utilization of cells subjected to one of two conventional formation procedures or the formation procedure according to the present invention; Fig. 4 is a line graph comparing the capacity of cells subjected to one of two conventional formation procedures or the formation procedure according to the present invention after the cells have been stored at high temperatures; Fie 5 is a line graph comparing the capacity of cells subjected to one of two conventional formation procedures or the formation procedure according to the present invention after the cells

have been stored for an extended period of time;

Fie 6 is a line graph comparing the capacity of cells subjected to one of two conventional formation procedures or die foimation procedure according to the present invention after the cells

have been subjected to a deep discharge;

Fie 7 is a line graph comparing the capacity of AAA cells subjected to one conventional formation procedure or the formation procedure according to the present invention; and

Fig. 8 is a line graph comparing the capacity of cells subjected to one of the formation

procedure according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As noted above, the present invention relates to an improved formation procedure for a NiMH electrochemical cell_. Such NiMH cells are rechargeable cells that include a metal hydπde ne_gative electrode and a positive electrode that includes a nickel compound that is capable of undergoing redox reactions, such as Ni(OH)2. In accordance with the present invention, d e positive electrode preferably includes a conductive foam carrier having a paste including the elecuochemicaUy active components, a binder, and a conductor, coated on and within the pores of the foam electrode carrier. The NiMH electrochemical cell may be assembled by any conventional technique_, such as spirally winding the positive and negative electrodes and separator layers and depositing the spiral wound electrode assembly (also known as a "jelly roll") into a cylindrical cell housing_. An alkaline electrolyte, which is preferably KOH, is also dispensed within the cell

housing prior to closure and sealing of the cell housing. The paste used to coat die positive elecuode includes a mix of Nι(OIT)2 and CoO The Nι(OH)₂ may be doped with Co, Zn, Al, Mn, etc The paste mix also preferably includes a conductor, a binder, and water The paste mix may be applied to the foam elecuode carrier using any conventional roll coating process The foam electrode earner may be made of any compatible Inehly conductive material and is preferably made of nickel The foam electrode preferably has between 80 and 1 10 pores per square inch

Once the electrochemical cell has been assembled, the cells are stored for a period of approximately one week at temperatures not exceeding room temperature During this preformation storage period, the CoO in the positive electrode mix dissolves in the alkaline electrolyte to form HC0O2 ions If the cells are stored less than three days at room temperature, the CoO m the positive electrode mix will not sufficiently dissolve in the electrolyte On the other hand, if stoied for more than one week, the HC0O2 ions tend to migrate to die separator and the negative electrode and also tend to readil) convert to die undesπable C03O in die presence of Inehly alkaline electiolyte wliich acts as an oxidizing agent By maintaining the storage temperature of die preformed cells at or below room temperature and mamiaming preformation storage between three days and one week, the conversion of die HC0O2 ions to C03O can be

effectively reduced

After the preformation storage peπod, the electrochemical cells are subjected to the inventive formation procedure, whereby the cells are first charged at a constant voltage Subsequently, the cells are charged at a first constant current level or at a second constant voltage Optionally, the cells may then be subjected to charging at a second constant current level that is lower than the fust constant current level The first of Uiese three charging steps is used to convert the CoO to CoOOH The second and third steps are used to convert the Nι(OH)2 to NiOOH According to a first embodiment, the first charging step is performed by setting the charging voltage to approximately 1 volt for three to five hours while allowing the current to vary between C/20 to 0 mA. The second step involves the charging of the cell at a constant current of C/3 for approximately four hours. The third step involves charging at a constant current level of C/10 for approximately two hours. By lowering the charging current in the third step, die Ni(OH)2. which may be buried deep in the electrode, is more likely to become activated than if the cell were continued to be charged at the C/3 constant current level utilized in the second charging step. Additionally, less oxygen is produced during the formation procedure when a lower current level is used in a final charging step. The inventive formation procedure thus takes a total of nine to ten hours to complete, which is a marked improvement over the thirty-two hour duration of some of the prior formation procedures. As will be described furdier below, the inventive formation procedure not only reduces the formation procedure time by more than 68 percent, the inventive formation procedure converts a significantly higher percentage of the Co(IT) in die cell to Co(HI). This increase in percentage cobalt conversion leads to better performance in terms of utilization, long-term storage, and deep discharge recovery.

To determine the advantages of the present invention, identical groups of NiMH cells were assembled and subjected to one of three different formation procedures. The first formation procedure (Procedure I) included the single charging step of charging at a constant current of C/20 for thirty-two hours. The second formation procedure (Procedure LT) included three charging steps all at constant current levels including a first step of charging at C/50 for five hours, a second step of charging at C/3 for five hours, and a final step of charging at C/10 for two hours. The third formation procedure (Procedure III) is that of the present invention whereby the cell was subjected to three charging steps with the first step being a constant voltage charge of 1.0 volt for three hours with the current allowed to vary between C/20 and 0 mA. The second charging step of the third formation procedure was to charge the cell at a constant current of C/3 for five hours. The third and last charging step was to charge at a constant current of C/10 for two hours. Fig. 1 shows the percent of Co(II) in the positive electrode that is convened to Co(LH) during the formation procedures (Procedures I, II, and HI). For this determination, three groups of three 4/3 A size cells with a capacity of 3.0 AH were formed using the three formation procedures. The illustrated percentages were computed by determining the amount of CoO added to the positive electrode mix (in grams) and multiplying that amount by 358 mAH/g, which is the electrochemical capacity of CoO. This number is the theoretical amount of Co(II) that could be converted to Co(III) in the positive electrode. Knowing that cobalt is converted from (II) to (III) during a charge at a constant current rate when the voltage has reached 1.1 volts, the length of time it took to reach a cell voltage of 1.1 volts was measured and multiplied by the charging current to yield the amount of Co(H) that was electrochemically converted to Co(IH) for those cells formed according to the constant current formation procedures (Procedures I and H).

For the cells formed according to the inventive formation procedure (Procedure III), the amount of CoQJ) that was converted to Co(IH) was determined by measuring and plotting the charging current versus time from commencement of the formation procedure until the charging current fell to 0.1 mA and then calculating the integral of the area under the plotted curve. To calculate the percentage of cobalt converted for all three procedures, the actual amount of cobalt converted was divided by the theoretical value and multiplied by 100.

As apparent from Fig. 1, the inventive formation procedure (Procedure III) results in a significant improvement in the percentage of Co(II) converted to Co(III). By converting a higher percentage of the cobalt during the formation procedure, less Co(H) is available to migrate through the separator and cause soft shorts and to form C03O4, which is electrochemically inert and acts as an insulator. The effects of convening more of the Co(H) to Co(IH) are described below with reference to Figs. 4-6. Fig. 2 shows the relation of the percentage of cobalt convened versus the charge current rate used for a constant current formation procedure (e.g., Procedures I and IT). While the percentage of cobalt converted generally increases with decreasing charge rates, the percentage of cobalt converted eventually levels off and does not approach the percentages of cobalt converted when utilizing the formation procedure of the present invention (Procedure IH). Furthermore, the lower the charging rate the longer it may take for the formation procedure.

Fig. 3 shows the effect of formation procedures on utilization. "Utilization" is the efficiency with which the active material of the cell is used or utilized during charge and discharge. To perform this test, three groups of three 4/3 A sized cells having a capacity' of 3 AH were constructed and formed using the three formation procedures. The percentages shown in Fig. 3 were calculated by dividing the actual capacity obtained during C rate discharge by the theoretical capacity of the cell and multiplying that value by 100. The theoretical capacity of the cell was calculated by multiplying the amount of Ni(OH)₂ (in gr.ams) in the positive electrode by 289 mAH/g, which is the electrochemical capacity of Ni(OH)2. The actual capacity obtained during C rate discharge was measured following the first charge after the formation procedure at a C rate to 38°C followed by a C/10 trickle for a total charge time of three hours. The C rate discharge was terminated when the cell voltage fell to 0.9 volt. The improvement in utilization is attributable to the improved conductive network surrounding the Ni(OHVNiOOH particles, which, in turn, is attributable to the increased amount of CoO (Co(II)) that is converted to CoOOH (Co(IU)) during the inventive formation procedure.

Fig. 4 illusuates the effect of the inventive formation procedure on the capacity of cells following storage at high temperatures. Three groups of eight 4/3 A sized cells were constructed and formed using the three formation procedures described above. Cells formed using the above three formation procedures were cycled nine times and then subsequently stored for 90 days at 40°C and then cycled while measuring their capacities. The results were plotted in the graph shown in Fig. 4. Cells subjected to the inventive formation procedure (Procedure IH) exhibited a marked improvement following storage at high temperatures. As compared to the capacities of the cells prior to storage at high temperatures, cells formed according to Procedure I exhibited a 6 percent drop in capacity from their initial capacity, while cells formed according to Procedure II exhibited a 1.8 percent drop in capacity'. Cells formed according to the inventive formation procedure, however, experienced no loss in capacity. Thus, the inventive formation procedure offers the advantage of significantly decreased ineversible capacity loss for cells stored at high temperatures. Fig. 5 illustrates the effect of storage time at 25 °C on cell performance for cells formed using one of the three formation procedures described above. Three groups of eight 4/3 A sized cells were constructed and formed using the three formation procedures described above. After formation, the cells were stored for nine months at 25 °C and then cycled to measure their capacities. As apparent from Fig. 5, cells formed using the inventive formation procedure have higher capacities than cells formed using the two conventional formation procedures after long-term storage. In terms of irreversible loss of capacity, cells formed using Procedure I experienced a 10.5 percent loss in capacity from their initial capacity, cells formed using Procedure II experienced a 2.6 percent loss, while cells formed using the inventive Procedure HI only experienced a 2 percent loss in capacity. Thus, die inventive formation procedure offers improved performance for cells that are likely to be stored in inventory or on retail shelves for prolonged periods of time.

Fig. 6 shows the effect of formation procedures on the performance of cells subjected to a deep discharge. Three groups of eight 4/3 A sized cells were constructed and formed using the three formation procedures described above. Following formation, the tested cells were initially cycled five times and then connected to a 23 -ohm resistor for one week to completely discharge the cells. This test was performed to replicate the over-discharge of cells as, at times, inadvertently performed by consumers. The tested cells were subsequently charged and discharged using the same conditions as used for initial cycling. Fig. 6 shows the capacity recovery after a deep discharge test. The capacities obtained from the various formation procedures were very similar before the deep discharge. After the deep discharge test, the average capacity of the cells formed using Procedure I dropped by 10.8 percent. The capacities of the cells formed using Procedure II dropped 6.8 percent, while the capacities of the cells formed using inventive Procedure III dropped 6.2 percent. To illustrate the effectiveness of the formation procedure of the present invention for an

AAA size cell, two groups of three identical AAA size, 600 rnAH cells were constructed. One group of cells was formed using formation Procedure II and the other was formed using Procedure III. The cells were then cycled and their capacities measured, averaged and plotted in the graph shown in Fig. 7. As shown in the graph, AAA cells formed using the inventive formation procedure exhibited higher capacities.

Like the formation procedure according to the first embodiment, the formation procedure of the second embodiment, includes a first charging step of charging the cell at a constant voltage of 1 volt. Thus, the same amounts of Co(II) are converted to Co(HI) for both embodiments. The second charging step differs, however, in that it involves charging at a constant voltage rather than a constant current to convert the Ni(OH)2 to NiOOH. The constant voltage at which the cell is charged in the second step is preferably 1.45 to 1.5 volts. The second charging step is preferably carried out for a predetermined period of time of, for example, between about five and nine hours. To determine the advantages of the inventive formation procedure according to the second embodiment, two groups of eight identical AAA size, 550 mAH cells were constructed. One group of cells was formed using Procedure HI above, while the other group of cells was formed using the following formation procedure, hereinafter referred to as "Procedure IV," which is the formation procedure according to the second embodiment of the present invention. First, the cells were charged at a constant voltage of 1.0 volt for three hours, and then were charged at a constant voltage of 1.5 volts for seven hours. The cells were then cycled and their capacities were measured and plotted in the graph shown in Fig. 8. As shown in Fig. 8, the capacities of cells subjected to Procedure IV are higher than that of cells subjected to formation Procedure IH.

The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the Doctrine of Equivalents.

Claims

CLAIMSThe invention claimed is:

1. A method of forming a NiMH electrochemical cell comprising the steps of:

(a) providing an assembled electrochemical cell having a positive electrode including a mix of a cobalt(TT) compound and Ni(OH)2; and

(b) charging the cell at a constant voltage.

2. The method as defined in claim 1 and further including the step of (c) charging the cell at a first constant current after performing step (b).

3. The method as defined in claim 2, wherein the charging in step (c) is performed for a predetermined time period.

4. The method as defined in claim 3, wherein the predetermined time period is about five hours.

5. The method as defined in claim 2, wherein the first constant current is about C/3.

6. An electrochemical cell formed by the method defined in claim 1.

7. The method as defined in claim 2 and further including the step of (d) charging the cell at a second constant current after the cell has been charged in steps (b) and (c).

8. The method as defined in claim 7, wherein the second constant current is lower than the first constant current.

9. The method as defined in claim 7, wherein the second constant current is about C/10_.

10. The method as defined in claim 7 wherein the charging in step (d) is performed for a predetermined time period.

11. The method as defined in claim 10, wherein the predetermined time period is about two hours.

12. The method as defined in claim 1 , wherein the constant voltage at which the cell is charged in step (b) is about 1 volt.

13. The method as defined in claim 1 , wherein the charging in step (b) is performed for a predetermined time period.

14. The method as defined in claim 13, wherein the predetermined time period is about three to five hours.

15. The method as defined in claim 1 and further including the step of (c) charging the cell at a second constant voltage after perforating step (b).

16. The method as defined in claim 15, wherein the charging in step (c) is performed for a predetermined time period.

17. The method as defined in claim 16, wherein the predetermined time period is about five hours.

18. The method as defined in claim 15, wherein the second constant voltage is between about 1.45 and 1.5 volts.

19. The method as defined in claim 1 , wherein said step of providing an assembled electrochemical cell includes providing a foamed positive electrode.

20. A method of forming a NiMH electrochemical cell comprising the sequentially performed steps of:

(a) providing an assembled electrochemical cell having a positive electrode including a mix of a cobalt(H) compound and Ni(OH ;

(b) charging the cell at a constant voltage;

(c) charging the cell at a first constant current; and

(d) charging the cell at a second constant current different from the first constant current.

21. The method as defined in claim 20, wherein the second constant current is lower than the first constant current.

2 The method as defined in claim 20, wherem the second constant current ⁱs about C/10

23 The method as defmed in claim 20, wherem die charging in step (d) ⁱs performed for a

prede_termmed time penod

24 The method as defined in claim 23 , wherem the predetermmed time period is about two

hours

25 The method as defined in claim 20, wherem the constant voltage at which the cell is charged

m step (b) is about 1 volt

26 The method as defined in claim 20, wherem the charging in step (b) is performed for a

prede_termmed time period

27 The method as defined in claim 26 , wherem the predetermmed time period is about four to

five hours

28 The method as defined claim 20, wherem the charging step (c) is performed for a

predetermmed time penod

The method as defined in claim 28, wherem the predetermmed time period is about four

29 hours

30. The method as defined in claim 20, wherein the first constant current is about C/3.

31. An electrochemical cell formed by the method defined in claim 20.

32. A method of forming a NiMH electrochemical cell comprising the sequentially performed steps of:

(a) providing an assembled electrochemical cell having a positive electrode including a mix of a cobaltCH) compound and Ni(OH)₂; (b) charging the cell at a constant voltage of about 1.0 volt; and

(c) charging the cell at a constant voltage of between about 1.45 and 1.5 volts.

33. The method as defined in claim 32, wherein the charging in step (b) is performed for a predetermined time period.

34. The method as defined in claim 33, wherein said predetermined time period is about three to five hours.

35. The method as defined in claim 32, wherein the ch^ging in step (c) is performed for a predetermined time period.

36. The method as defined in claim 35, wherein said predetermined time period is about five to nine hours. PCMJS99/20173

37. An electrochemical cell formed by the method defined in claim 32.

38. An electrochemical cell comprising: a metal hydride negative electrode; a positive electrode including NiOOH and cobalt compounds where more than 81 percent of the initial cobalt present in said positive electrode is converted to cobalt (III); and an electrolyte for transporting ions between the positive and negative electrodes.

39. The electrochemical cell as defined in claim 38, wherein at least about 93 percent of the initial cobalt present in said positive electrode is converted to cobalt (III).

40. An electrochemical cell comprising: a metal hydride negative electrode; a positive electrode including NiOOH and cobalt compounds wherein the percent utilization in said positive electrode is greater than 98 percent; and an electrolyte for transporting ions between the positive and negative electrodes.

41. The electrochemical cell as defined in claim 40, wherein the percent utilization in said positive electrode is at least about 105 percent.